Adjust update_prevalence Canada data approach to use fewer API calls

[blanchardjeremy]

The current limits of the covid19-tracker.ca API require us to make 100+ API calls and then we run into throttling errors on the API.

Two options that I currently see:

• Either wait until this issue is handled by our current API: https://github.com/andrewthong/covid19tracker-api/issues/88
• Or consider switching to a different API as outlined here: https://github.com/microcovid/microcovid/issues/956#issuecomment-897140392

[blanchardjeremy]

Tagging @jy2wong - looks like they updated the API with your request. (though it appears to be set to only 7 days. But I haven't tested it yet.)